1. Field of the Invention:
This invention relates to the reactor vessel internals of a nuclear reactor, and more particularly to a baffle and former assembly. The assembly alleviates the potential for detrimental overpressurization in the assembly under assumed major accident conditions, and reduces coolant cross flow impingement on the reactor fuel assemblies, by utilization of specially located formers and flow holes.
2. Description of the Prior Art:
A typical liquid cooled nuclear reactor includes a singular reactor vessel, housing the heat generating reactor core, and a plurality of flow loops through which the reactor coolant fluid is circulated. In each loop, coolant which is heated in the core typically is placed in heat exchange relation with a vaporizable fluid which is used to drive turbine-generator apparatus. The coolant is then recirculated to the reactor vessel. Within the vessel are the reactor vessel internals, the functions of which include support of the core components, such as the fuel assemblies, guidance of reactor coolant flow, and support of core monitoring apparatus. Most of the supported load is transmitted through the wall of the massive core barrel which surrounds the reactor core. Coolant flow generally enters the vessel, passes downward in an annulus formed between the barrel and vessel, and then is turned 180.degree. to flow up through the core and out of the vessel. Between the core barrel and the core is typically a baffle plates-and-formers assembly, also supported by the core barrel, which confines and directs flow into the core region and also provides an annulus that shields the core barrel wall from excessive irradiation. The baffle plates abut against one another about the core, and gaps may form between the plates due to differential thermal expansions among the internals components. Coolant circulating through the baffles-formers assembly may therefore undesirably pass through the gaps and impinge upon the core fuel assemblies, resulting in detrimental local loadings on the assemblies.
Some internals designs have been based upon downward flow in the baffle-former annulus, the coolant entering at the upper portion of the annulus, passing vertically through holes in the formers, and exiting to turn 180.degree. and pass through the core. Such designs have the advantage of alleviating core bypass flow, which bypass flow lessens the thermal efficiency of a reactor. However, some flow may still leak through the gaps between adjacent baffle plates into the core due to the high pressure differential between the baffle region and core, undesirably impinging in a cross flow fashion upon the fuel assemblies. Such cross flow can induce unacceptable fuel assembly vibration. More recent designs have therefore incorporated upward flow in the baffle-former annulus, such that in addition to the large flow of coolant upward through the core, a relatively small bypass stream passes in parallel through the annulus. This reduces the pressure differential and alleviates the tendency for leakage through the baffle plates. However, this design raises concerns with respect to the amount of flow through the annulus. It is desirable to minimize the flow, as its bypassing the reactor core results in lower reactor thermal efficiency. If the annulus is kept relatively open, by utilization of a large area of flow openings in the formers, an unacceptable bypass in excess of one to two percent could result. Minimizing the flow rate to a range which still provides adequate baffle, former, and barrel cooling, on the order of one-half percent of total flow during normal operation, raises concerns under assumed design-basis accident conditions. In the unlikely event of a rupture of the main coolant piping in one of the circulating loops, a rapid depressurization of the reactor system results, referred to as "blowdown". Under such conditions, the coolant in portions of the vessel will depressurize and flash to steam, including the coolant in the baffle-former annulus. With a baffle-former flow area that has been limited in order to increase efficiency, the core will depressurize faster than the annulus, and the flashing coolant could build up excessive pressures in the annulus. The overpressurization can damage the baffle assembly and also the adjacent fuel assemblies, potentially failing the fuel rods.
It is therefore desirable to have a baffle-former assembly which overcomes these deficiencies in the prior art and effectively incorporates the pressure differential benefits provided by an upward flow in the baffle-former annulus, the efficiency benefits provided by minimizing core bypass flow, and which further will eliminate the overpressurization concerns under assumed accident conditions.